The utilization of geothermal heat is regarded as belonging to the regenerative energies. The temperature rises with increasing depth. This increase in temperature is not dependent on seasons or climate, but solely depends on geological and geothermal conditions. For this reason, the utilization of geothermal heat for energy production is a good option in many regions because emission-neutral, especially with respect to CO2 emissions, and safe production of energy is possible in this way.
Various systems and processes for the utilization of geothermal heat are known from the prior art. The most widely used method of utilization involves a geothermal heat probe, such as disclosed in DE 29 35 832 A1. Therein, a U-pipe is introduced in a borehole in the soil. A liquid circulating in the tube absorbs heat from the environment in the depths, which heat is subsequently utilized. A pump is provided to support circulation. This solution is disadvantageous in that the efficiency and performance of the system are limited for constructional reasons.
Groundwater offers another possible way of utilizing geothermal heat by conveying the groundwater through an extraction well to the surface where heat is withdrawn therefrom by means of a heat pump. The water is subsequently fed back into the aquifer through an injection well. This solution is disadvantageous in that the extraction of groundwater requires permission for which fees must be paid, reinjection involves monitoring obligations, and depression or rise of the groundwater table results each time. The chemical and physical parameters of water withdrawn from the extraction well frequently differ from those in injection wells just a few meters away so that chemical reactions and precipitation reactions take place, blocking the well in the long run. In addition, the pressure differences occurring during above-ground pumping involve the risk of out-gassing of dissolved gases and precipitation reactions associated therewith.
EP 0 386 176 B1 discloses a system for exchanging energy between the soil and an energy exchanger via a combination of a forward pipe with a pump in the borehole and a feedback pipe. The borehole is provided with a porous filling, and water is introduced into the borehole through the forward pipe to reach the feedback pipes through the porous filling. The feedback pipes are provided with a combination of transverse seals and through-openings in the direction of the porous filling so that the water, when conveyed to the surface, is always forced to leave the feedback pipe. The special configuration of the feedback pipes is intended to increase the heat absorption of the water. This solution is disadvantageous in that implementation thereof with sufficient efficiency is only possible over long lengths.
EP 0 755 497 B1 discloses a system for extracting geothermal heat, wherein water is introduced down to the bottom of the bore in the outer region of the borehole. A shroud pipe is arranged at a defined distance to the bottom of the bore, which pipe has a pump in the lower region thereof, the pump being intended to convey water to the soil surface. The region of the bore between the outlet opening of the water-supplying pipes and the lateral opening of the shroud pipe is provided with a porous filling. Although the inventive measures of EP 0 755 497 B1 are intended to take up preferably warm water from the lower region of the bore, the existence of a hydraulic connection between supplying and discharging pipes is disadvantageous, so that preferably cold water is conveyed to the surface which has previously been introduced into the borehole. As a result, the efficiency decreases considerably.
Other prior art systems have been described in EP 1 388 717, JP 58024762, U.S. Pat. No. 3,938,592, DE 20 2004 016 998, DE 2850865 and CH 653120.
EP 1 388 717 discloses a system for utilizing geothermal heat, which system has a main pipe divided into an upper part and a lower part by a transverse seal. The pipe has through-openings towards the surroundings. The pipe has a pump arranged therein which conveys groundwater upwards through heat exchanger pipes. Pipes are arranged in the main pipe for circulation of a heat carrier. This invention requires large bore calibers and does not provide elastic and hydraulically advantageous embedding in porous material.
JP 58024762 describes a method for extracting geothermal heat using a main pipe provided with through-openings upstream and downstream of a transverse seal. Therein, withdrawal from a groundwater-bearing layer and introduction into a hydraulically separated, different aquifer have been depicted as being essential and fundamentally necessary. This results in mixing of different groundwaters usually having different chemical and physical water qualities, e.g. bearing freshwater and saltwater, which may give rise to precipitation reactions and blocking of the well filter sections in the long run. Also, mixing of different groundwaters is prohibited in most regions for groundwater protective and ecological reasons and involves problems in regions with groundwater utilization for drinking water production.
U.S. Pat. No. 3,938,592 discloses a system for utilizing geothermal heat, which system has a main pipe divided by a transverse seal having a pump arranged therein. A U-shaped water exchange pipe for circulation of a heat carrier is arranged upstream of the transverse seal, the main pipe being surrounded by a porous bed. The installation requires relatively large underground hollow spaces through multiple drillings or blastings and is therefore not applicable in loose rock normally due to the instability of the foundation.
The disclosures of DE 20 2004 016 998, DE 2850865 and CH 653120 describe devices for the utilization of geothermal heat with a heat exchanger for direct evaporation of a liquid coolant, or they disclose main pipes or heat exchanger pipes made of PVC or polyethylene. As a result of the, at best, very slow groundwater flow, the underground closed heat exchanger surfaces will merely allow transfer capacities virtually at the level of pure conduction of heat.
Solutions known from the prior art preferably use separate pipes in the borehole to extract and return the groundwater, as well as a separate heat exchanger in the form of a separate system. As a result, the systems known from the prior art are complex and cause high expenses due to the requirement of larger bore diameters.
In particular, the problems with sealing or interaction of the main pipe with the surrounding soil have not been solved in the prior art so that prior art systems are operating with insufficient efficiency.